The invention is directed to mouldable and vulcanisable rubber mixtures based on halogen free rubbers which contain as essential constituents besides the rubber which contains no halogen in the molecule, a silicate filler, optionally in admixture with the filler carbon black, optionally sulphur and/or sulphur donor, at least one vulcanisation accelerator and at least one organosilane with at least one alkoxysilyl group.
It is known to add a silica filler in place of carbon black in rubber mixtures based on so-called All-Purpose rubbers (for short AP rubbers, i.e. types of rubbers which have found the widest use for the best known areas of use as especially styrene-butadiene rubber, butadiene rubber, butadient-acrylonitrile rubber and natural rubber) limited by the following three property conditioned grounds.
1. In AP rubbers silica fillers at comparable specific surface areas as the carbon blacks produce considerably higher Mooney viscosities than carbon black.
2. Silica fillers negatively influence the vulcanisation kinetics and the crosslinking efficiency of the preferably employed accelerated sulphur vulcanisation with AP rubbers.
3. Silica fillers in rubber mixtures and vulcanisates have smaller effective rubber surface areas which are very strongly expressed in reduced efficiency, especially in abrasion resistance.
It is known that these important disadvantages can be overcome by the use of organosilanes in the rubber mixtures filled with silica fillers. Such organosilanes must be "bifunctional", i.e. have first a filler active function which customarily is exercised by alkoxysilyl groups and secondly have a rubber active function, which customarily is exercised by sulphur containing groups as the --S.sub.x --(X=2 to 6) and the --SH groups. The last mentioned groups obviously share in the accelerated sulphur vulcanisation reactions.
Well suited organosilanes for example are the outstanding suited bis-(alkoxysilylalkyl)-oligosulphides as for example bis-(triethoxysilylpropyl)-tetrasulphide (German patent 2,255,577 and related Thurn U.S. Pat. No. 3,873,489). An alternative to the separate addition of silicate fillers and silanes to the rubber mixtures consists of the premixes of the mentioned materials (Thurn U.S. Pat. No. 3,997,356). (The entire disclosures of both Thurn U.S. patents are hereby incorporated by reference and relied upon.) In this manner there are formed silica containing vulcanisates which produce even in high requirement articles such as tire tread surfaces performances equivalent to carbon black. A disadvantage of the mentioned compounds is the multistep production, thus the high industrial expense, which is also reflected in correspondingly high prices.
There is already known a cross-linkable rubber mixture which contains an oligosulphidic organosilane, known vulcanisation accelerator and as filler a silicate filler, but contains no elemental sulphur (Belgian Pat. No. 832,970 or German Pat. No. 2,356,674 or related Wolff U.S. patent application Ser. No. 34,203 filed Apr. 27, 1979. (The entire disclosure of the Wolff U.S. application is hereby incorporated by reference and relied upon.) The crosslinking carried out therewith can be designated simply as "sulphur free silane" crosslinking.
For mixtures based on SBR (styrene-butadiene rubber) and EPDM (ethylene-propylene-diene) rubbers there are also already known mercaptosilanes such as 3-mercaptopropyl trimethoxysilane, vinyl silane as well as vinyl trimethoxysilane and aminosilanes such as 3-aminopropyltriethoxysilane (Rubber World, October 1970, pages 54-54, the entire disclosure which is hereby incorporated by reference and relied upon).
There have also already been proposed silicate filler containing rubber mixtures based on the selected group of the halogen containing types of rubbers in which the mixtures inter alia contain specific halogen containing silanes producible in simple manner and easily available. It has been surprisingly shown that these mixtures give very valuable vulcanisation products.